In recent years, global warming due to an increase in the amount of emission of carbon dioxide is a problem. Even in the steel industry, reducing the amount of emitted CO2 is an important issue. Therefore, in recent operations of blast furnaces, low reducing agent rate (low RAR) operations are greatly encouraged. (The reducing agent rate is the total amount of reducing agent injected from a tuyere and coke charged from the top of a furnace, per 1 ton of pig iron that is manufactured). In a blast furnace, coke and pulverized coal injected from a tuyere are primarily used as reducing agents. To achieve a low reducing agent rate and, thus, suppress the amount of emission of carbon dioxide, it is effective to replace, for example, coke with a reducing agent having a high hydrogen content such as waste plastic, LNG, and heavy oil. Japanese Unexamined Patent Application Publication No. 2006-291251 discusses that, when two or more lances that inject reducing agents from a tuyere are used and a flammable reducing agent such as LNG, and a solid reducing agent such as pulverized coal, are injected from different lances, the lances are disposed so that an extension line of a lance that injects the flammable reducing agent and an extension line of a lance that injects the solid reducing agent do not cross each other. According to Japanese Unexamined Patent Application Publication No. 11-241109, when a lance that supplies a reducing gas is disposed in front of, that is, closer to a blast furnace side by 50 to 10 mm in a injecting direction than a lance that supplies a solid reducing agent such as pulverized coal, pressure loss at an end of a tuyere and a blow pipe is reduced so that stability of a furnace condition is increased.
Although, compared to a conventional method of injecting only pulverized coal from a tuyere, the method of operating a blast furnace in Japanese Unexamined Patent Application Publication No. 2006-291251 has the effect of increasing combustion temperature and reducing a unit consumption of reducing agent, it can be further improved. In the method of operating a blast furnace in the Japanese Unexamined Patent Application Publication No. 11-241109, since the reducing gas is not sufficiently preheated/its temperature is not sufficiently raised, the effect of raising the temperature of pulverized coal due to the formation of a combustion field is small, and oxygen at a point where the pulverized coal is ignited and starts burning is consumed, as a result of which the combustion of the pulverized coal may be hindered.
It could therefore be helpful to provide a method of operating a blast furnace that makes it possible to further increase combustion temperature and reduce unit consumption of reducing agents.